Over the last two seasons he has 11 homers in 479 at-bats.
homers aren't the only form of power, plus more specifically(I am only talking about using him in either spot starts vs LHP or off the bench vs LHP):
From 2005-2007 he has 233 at bats vs lefties and put up:
283 .387 .468
He had 66 hits, 28 of which were for extra bases, 20 doubles, 7 homers, 1 triple. that's 42% of his hits vs lefties that went for extra bases.
These numbers include his poor 2007 vs lefties of .221 .323 .372.
But if we go back to 04 here was his line vs LHP: 267 .366 .461
03: 301 .403 .515
02: .225 .275 .345 .620
So he had 2 bad years, in 2002 and last year vs lefties, but he had 4 straight very solid years in between. He struggles vs righties, and if last yr was an aberration not a sign of declining skills, he should be fine as a right handed power option vs lefties on the bench.